The invention relates to an optoelectronic device for acquisition of images, in particular of bar codes.
The present optoelectronic devices which are designed for reading of bar codes conventionally comprise a case which contains an electronic scanning sensor, optical means which make it possible to form images on the sensor, and to define an optical plane together with the latter, and electro-luminescent diodes which are designed to light the bar codes, and to permit acquisition of the images on the sensor. Finally, these devices comprise processing electronics which permit piloting of the sensor, and processing of the signals obtained from the latter.
Most of the present optoelectronic devices are provided with electro-luminescent diodes which are offset relative to the optical plane, so as not to interfere with the beams which are reflected by the bar code. Devices of this type, which are described in particular in patents U.S. Pat. Nos. 5,397,885, 4,408,120 and EP 101 939, have the disadvantage that they have errors of parallax which, in practice, lead to limitations of their possibility of use, and in particular to a very short permissible reading distance.
In addition, the arrangement of the electroluminescent diodes of these devices is such that the beams of the various diodes overlap only very partially.
Firstly, the brightness of the diodes conventionally used is highly disparate, and in fact the class of brightness of these diodes can vary between 1 and 4. As a result of this disparity, when the diodes age, it is found that there are local attenuations of the brightness intensity, which are not distributed and correlated throughout the width of the field of reading, and can lead to lack of recognition of the bar codes, derived from extraction of binary signals which do not emerge from the noise threshold of the processing electronics.
Secondly, this arrangement of the diodes leads to reduction of the performance of the device, when any one of the said diodes burns out.
Finally, for most of the present optoelectronic devices, the arrangement of the diodes leads to lighting of areas on both sides of the bar codes, which are beyond the useful field of reading. This gives rise to an ergonomic problem, consisting of the difficulty of positioning the device adequately opposite the bar codes, in particular if the latter are relatively long. This then also results in a problem of loss of energy.
The optoelectronic device which is described in patent application FR-2 673 738 makes it possible to eliminate one of the disadvantages of the aforementioned devices, i.e. the disadvantage relating to the errors of parallax. In fact, this device comprises electro-luminescent diodes which are disposed substantially in the optical plane, outside the optical field of reading, and are designed to create beams which cover a band with a height of between 1 and 6 mm on both sides of the optical plane. As already stated, a device of this type, the diodes of which are disposed in the optical plane, makes it possible to avoid errors of parallax, and consequently permits distance reading of the bar codes. On the other hand, it has the other aforementioned disadvantages relating to lack of overlapping of the beams of the diodes, and lighting of areas which are outside the field of reading.
Other optoelectronic devices, such as described in international patent application WO-14471 and patent EP-524349, also make it possible to eliminate the disadvantage relative to the errors of parallax.
For this purpose, according to these devices:
the lighting means are disposed downstream from the optical means, and are disposed such that the axis of the light beam intersects the optical axis;
reflection means which can allow the image beam to pass through, and can reflect the light beam, are disposed such as to intercept the said light beam;
the said reflection means are inclined relative to the optical axis, by an angle which is such that the light beam is centred on the said optical axis; and
the said reflection means, lighting means and lens are disposed such that the distance between the object and the lens is substantially identical to the distance between the object and the lighting means.
However, as in the previous cases, they do not make it possible to solve the aforementioned disadvantages relating to lack of overlapping of the beams of the diodes, and to lighting of areas outside the field of reading. In addition, they give rise to an additional disadvantage, since the reflection means consist of semi-reflective mirrors. Reflective mirrors of this type lead to recovery of 25% at the most of the lighting energy emitted, at the level of the image. Consequently, they require use of powerful lighting sources which are incompatible with autonomous devices.
The object of the present invention is to eliminate all of the above-described disadvantages of the present optoelectronic devices, and its main objective is to provide an optoelectronic device which combines the following advantages: substantial depth of field, regular reduction of the light beam across the width of the useful field of reading, clear delimitation of the edges of the lit area, and low electrical energy consumption.
For this purpose, the invention relates to an optoelectronic device for acquisition of images of objects, in particular of bar codes, comprising a case which is provided with a reading window and contains an electronic scanning sensor, lighting means, and optical means which permit the formation of images on the sensor and comprise a lens, the sensor and the optical means defining an optical axis relative to which the reading window is centred, the said sensors, reading window and optical means delimiting a useful optical reading field. This optoelectronic device comprises the characteristics previously described (with reference to WO-14471 and EP-524349), and, according to the invention, the following characteristics:
the lighting means comprise a lighting source which is designed to light the useful optical field throughout the entire depth of the field;
the angle xcex1, from which the lighting source is seen from the object plane, is such that       α    ≤          Arc      ⁢              xe2x80x83            ⁢      tg      ⁢              L                  5          ⁢          P                      ,
in which L=width of the object plane, and
P=the distance between the object and the lighting source.
An optoelectronic device of this type is designed to obtain:
a light beam which is designed to light the optical reading field throughout the depth of the field C of the said device, and thus approaches the light beam which is emitted by a point source. (It should be noted that the depth of the field C conventionally means the reading range which is delimited by the maximum and minimum distances of work of the device, which depend on the performance of the optical and electronic processing means); and
superimposition of the vertices of the angle of emission of the lighting source and of the optical reading angle.
In fact, as a result of the arrangement of the lens and of the lighting source:
the vertex of the angle which subtends the optical reading field is on the entrance pupil of the lens; and
the lighting source, the main axis of which is co-axial relative to the optical axis, has an angle of emission, the virtual image of the vertex of which, seen from the object, coincides substantially with the entrance pupil of the lens.
These two operating characteristics, which make it possible to obtain quasi-superimposition of the lighting area and of the optical area, provide the following advantageous results:
regular reduction of the lighting of the optical field, if the brightness of the lighting source decreases over a period of time, and consequently lack of local attenuation of the light intensity, throughout the depth of the field. As a result, the performance of the device is more stable over a period of time, and consequently the reliability of the latter is also more stable over a period of time;
accurate delimitation of the optical area, as a result of the superimposition of the lit area and the reading area, and consequently sharpness of the edges of the lit area; and
increased depth of field, as a result of the lack of parallax error, which depth is limited only by the sensitivity of the sensor and of the lighting power, and not by the parallax faults.
According to another characteristic of the invention, the reflection means comprise a reflective surface which is provided with a slot which is centred on the optical axis, and is disposed such as to intercept the light beam, the said reflective surface being inclined relative to the optical axis, by an angle which is adapted such that the light beam reflected is centred on the said optical axis.
An arrangement of this type makes it possible to recuperate a maximum, and approximately 80% of the lighting energy emitted. In fact, the reflection means are provided with an aperture which is used by the optical path. It should however be noted that lighting means of this type produce a beam which is slightly convergent vertically. In fact, the reflection means break the light beam down into two secondary light beams, which are non-coplanar relative to the optical plane. However, the slot is sufficiently narrow to introduce a very slight parallax error, such that the lit area corresponding to the depth of field is sufficient.
In addition, in order to minimise the loss of energy of the light beam, the reflective surface has vertically in its median portion the shape of an S, which is provided with a substantially vertical wing, in which the slot is provided.
In addition, this reflective surface is preferably articulated on a fixed support, by means of an axial hinge system, and adjustment means are disposed above the said reflective surface, such as to be supported on the latter, and to permit adjustment of its inclination, and/or to make it pivot.
These adjustment means additionally also consist of two screws, which are disposed perpendicularly relative to each of the longitudinal ends of the reflective surface.
In addition, in order to avoid any risk of warping, the reflective surface comprises an upper stiffening edge.
According to another characteristic of the invention, the lighting means comprise at least two diodes which are disposed one in the vicinity of the other, or which are disposed in the vicinity of each other, such as to obtain substantially total overlapping of the light beams of each diode throughout the entire depth of the field.
According to this characteristic, the point source is approached by using at least two point sources, which are disposed such as to obtain a relatively regular light beam, which, as previously stated, leads to regular reduction of the lighting when the diodes age.
In addition, in this case, the device according to the invention makes it possible to eliminate disparities of brightness of the diodes, as a result of the substantially total overlapping of the beams of the diodes.
Finally, according to this characteristic, even if a diode burns out, the device can continue to be operational, since the loss of intensity is applied substantially regularly over the lighting field as a whole, rather than locally.
According to another characteristic of the invention, the lighting means comprise diodes which have an angle of diffusion which is greater than the optical angle, and associated optical means which are disposed such as to intercept the light beam of the said diodes, and are designed to make the said light beam converge in the direction of reduction of the angles of emission, such as to reduce the latter to a value which is conjugated with that of the optical angle.
This preferred embodiment makes it possible to obtain an optoelectronic device which combines the aforementioned advantages, by means of electro-luminescent diodes which are of a conventional type, and thus have a low cost price. An optoelectronic device of this type is particularly suitable for permitting reading of images at distance of approximately 5 cm to 50 cm, and has increased sensitivity owing to the presence of optical means which are associated with the diodes, which focus the maximum light intensity onto the optical reading field.
According to another characteristic of the invention, the optical means which are associated with the diodes comprise a convex focussing lens, which is designed to intercept the light beam for the diodes as a whole.
In addition, this focussing lens preferably comprises a convex dioptre with a toroidal shape. (By toroidal shape is meant a dioptre which conventionally has a radius of curvature in a first plane which is parallel to the light beam, and a radius of curvature which is much smaller, according to a plane which is at right-angles to the said first plane).
As well as permitting focussing of the light beam according to a plane which is at right-angles to the bar code, as previously stated, this toroidal shape of the dioptre makes it possible to focus this beam according to a direction which is parallel to the bar codes, and thus to obtain a thin light beam which is centred on the optical axis.
According to another characteristic of the invention, the focussing lens is a flat-convex lens, and comprises a flat dioptre which is provided with a recess designed to accommodate the electro-luminescent diodes.
In addition, the recess of the flat dioptre of this lens is preferably filled with a resin which has the same refraction index as the said lens.
This arrangement has the advantage that it provides a global general lighting angle, which is defined by the additional dioptre, and is substantially equivalent to the lighting angles of the diodes.
In fact, owing to the presence of the resin, the light beams which are provided by the chips of the diodes do not undergo any deflection upstream from the convex dioptre of the lens.